This disclosure generally relates to gas turbine engines, and more particularly, to combustors for gas turbine engines.
Microturbines are small gas turbines typically used for on-site power generation. They operate on the same principle as a jet engine but can use a variety of commercially available fuels, such as natural gas, diesel, bio-diesel, gasoline, kerosene, propane, methane, digester gas, reformed fuels, products of gasification and the like. Microturbines have the ability to operate in grid-connected, stand-alone, and dual modes. Grid-connected mode generally allows the unit to operate parallel to the grid, providing base loading and peak shaving. Stand-alone mode generally allows the units to operate completely isolated from the grid. In dual mode, the units can switch between the two modes automatically.
Microturbines are generally applied as back-up or auxiliary power sources for office buildings, retail stores, small manufacturing plants, homes, and many other commercial facilities. These facilities have traditionally been powered by electric utilities via a grid of power distribution lines. Using microturbines, these facilities generate electrical power at their own sites, and avoid being solely dependent on conventional electrical power grids and utilities. Microturbines may also generate power at less cost and/or more reliably than the electrical power provided over the grid by electrical power utilities.
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (UHC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of the high combustor flame temperatures during operation.
A conventional microturbine generally includes a compressor, a recuperator, a combustor, and a turbine. Air is compressed in the compressor, heated in the recuperator, mixed with fuel, burned in the combustor and then expanded in the turbine to generate hot, high-pressure gases that drive the turbine. The turbine exhaust gases are generally ducted through the recuperator to transfer heat to the inlet air and thereby increase the energy of the air-fuel mixture in the combustion chamber. There are generally two types of combustors employed with gas turbines, e.g., can-type combustors and annular-type combustors, each having characteristic advantages and disadvantages relating to emissions and operability.
Can-type combustors typically consist of a cylindrical can-type liner inserted into a transition piece with multiple fuel-air premixers positioned at the head end of the liner. Although this system is practical and easy to assemble, prior art can-type combustors have several inherent disadvantages for achieving ultra-low emissions and maximum operability. Prior art can-type combustors are relatively lengthy and provide a long combustor residence time. During low load and/or low temperature operation, the levels of CO and UHC are minimized due to the long combustor residence time. However, during high load and/or high temperature operation, diatomic nitrogen begins to react with combustion intermediate species (O-atoms, OH, etc), and NOx emissions grow in time. Therefore, the large residence time of the can-type combustor results in high NOx emissions during high-load and/or high temperature operation. In contrast to high load and/or high temperature operations, at lower pressures and similar flame temperatures, the CO levels increase significantly unless the residence time is increased. This is particularly important for operations on microturbines, which have much lower pressure ratios (typically around 4.0) than large machines. As a result, combustors need to be modified accordingly for implementation on recuperated microturbines.
FIG. 1 illustrates a prior art can-type combustor. The can-type combustor shown generally by reference numeral 2 includes a casing 4, premixing means 6, air inlet(s) 8, a can liner 10, a combustion chamber 12, an optional transition piece 14, and a nozzle 16. During the operation of prior art can-type combustor 2, combustion air enters in through air inlet(s) 8 along the direction of arrows A and enters into casing 4. Combustion air then enters premixing means 6 where it is mixed with fuel. The fuel-air mixture is then injected by premixing means 6 into combustion chamber 12 where it is combusted. After the fuel-air mixture is combusted it is exhausted through transition piece 14 and nozzle 16. As mentioned, one down side of the can combustor is its length. The combustion products flow from the upstream end of the combustion chamber through the entire chamber and enter into the transition piece until exiting through the nozzle. This results in a long combustor residence time and accordingly, during high temperature and/or high load operation, high levels of NOx emissions. However, the can-type combustor works well during low temperature and/or low load operation, as the long combustor residence time allows the CO and UHC to burn off (i.e., oxidize more completely) during this long period, resulting in low CO and UHC emission levels. In addition, particularly for microturbine combustors, the transition piece is usually not required since the combustor exit may be aligned with the scroll inlet and any leakage minimized via the use of a seal.
Annular-type combustors typically consist of multiple premixers positioned in rings directly upstream of the turbine nozzles in an annular fashion. The annular-type combustor is short in length and accordingly, has a relatively short combustor residence time. During high load and/or high temperature operation, the levels of NOx emissions are low due to the short combustor residence time in the short annular combustor. However, during low load and/or low temperature operation, the levels of carbon monoxide (CO) and unburned hydrocarbon (UHC) are large due to the short combustor residence time of the annular-type combustor, not allowing complete CO and UHC burnout (i.e., oxidation).
FIG. 2 depicts a prior art annular-type combustor, generally designated by reference numeral 50. As shown, a typical annular-type combustor 50 consists of a single flame tube, completely annular in form, which is contained in a continuous, circular inner and outer combustion casings 52, 54, respectively, without any separate interior burner cans. This construction provides the most effective mixing of fuel and air, and due to optimum burner surface area, maximum cooling of the combustion gases takes place. Due to its annular shape, the annular-type combustor has no need for a transition piece, making it much more compact than a can type combustor. As discussed earlier, one down side of the annular combustor when implemented for low-pressure ratio gas turbines is this short length. The combustor residence time is low, and accordingly, during low temperature and/or low load operation, high levels of CO and UHC emissions are present. In addition, one other important down side of annular combustors operation is the multitude of acoustic modes of the combustion system (transversal and longitudinal), which are especially prone to excitation in the case of lean premix flames, and may therefore result in high amplitude pressure fluctuations, generally at high loads. However, the annular combustor works well during high temperature or high load operation, as the short combustor residence time does not give the NOx emissions sufficient growth time, resulting in low levels of NOx emissions. Moreover, for microturbines, the use of radial flow turbomachinery is normal whereas the geometrical layout of an annular combustor is best suited for axial flow turbomachinery.
In microturbine engines, usually a lean premixed flame is employed. In can-type combustors for microturbines, this is achieved by using a premixer that performs a dual operation role for generating the premixed and diffusion flames. The latter is usually employed in conditions other than the design point (full speed load), where stabilization of a premixed lean flame is generally difficult to achieve. In annular type combustors, a circumferentially uniform array of premixers is employed. Furthermore, all of the premixers are operated similarly for achieving uniformity and good pattern factors. The result is that, in either premixer configuration (e.g., annular or can type,) higher emissions (whether it be CO and UFHC as in the case of annular type combustors or NOx as in the case of can-type combustors) occur at conditions other than the design set point and that no flexibility is permitted in either premixer configuration to operate the premixers independently at different fuel rates.
Accordingly, there is a need for quiet combustors that minimize emissions at low temperature and/or low load operation as well as well during high temperature or high load operation (i.e., emissions at design set point as well as emissions at operation conditions other than design set point).